Let's see, today I'm going to take my 45 acp suppressor and modify it to use with 5.56.....308......(you get the point).

No modification required. If it will thread on to the muzzle, you can put a larger than firearm caliber suppressor on your gun... so, yes, a .45 suppressor will work, albeit not very well in some cases, on any firearm .45 cal or less.

To expand on this, a suppressor intended for a 7.62 works fairly well on a 5.56. No... not as well as a dedicated 5.56 unit, but still, pretty quiet. No dB meter data to back up what I'm saying... just what I and others heard.